High speed x-ray pulsing device

ABSTRACT

An x-ray tomography apparatus has a couch for a human subject to lie on, a cylindrical capsule to enclose part of the body, a flexible water filled bag to stabilize a portion of the human body being x-rayed, an x-ray source mounted on a rotatable frame, and an arcuate row of x-ray detector plates enclosed in a metal container filled with xenon gas. The detector plates are positioned on the rotatable frame opposite the x-ray source so that the x-rays pass through the subject and are received by the detector plates. The detector plates and x-ray source are continuously rotatable about the subject, having slip ring power and electrical connections. The couch is servo motor powered and is switched at the completion of one rotation of the rotatable frame to automatically index the subject forward. By continuous rotation of the x-ray source and detector plates and automatic indexing of the subject the continuous operation of the tomograph is possible and multiple observations on the subject can be made in a short time. The x-ray data obtained can be analyzed mathematically, e.g., by a computer program, and displayed in visual or numerical form by conventional techniques.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention is related to apparatus for examining an object by meansof radiation such as xor gamma radiation. More particularly, theinvention is concerned with tomographic rotational examination whichenables two dimensional x-ray examination to represent the threedimensional configuration of a thin tomographic slice. Tomography hasbeen employed to examine parts of the human body, such as the head, toascertain the location of tumors and extraneous or foreign matter.

Some of the prior art tomographic devices provide for rotation of anx-ray source and x-ray detectors about the portion of the object beingexamined. Some of the prior art devices which utilize rotation employcables which limit the extent of possible rotation, and thus necessitatethe expenditure of time and effort by a technician to readjust orreposition the cables so that the x-ray process can be continued.Rotation may be limited to as little as 180° in some devices. In otherprior art devices, it is necessary for the x-ray technician or operatorto manually adjust the position of the human body so that a differentportion of the body can be x-rayed by a scan after a previous sectionhas been scanned. Still other prior art devices have an x-ray source andx-ray detectors which are moved reciprocally relative to the bodyportion being examined, with their positions relative to the bodyportion being changed after each scan is taken.

Prior art devices have also taken x-ray slices of thicknesses of 8millimeters or greater. The thickness of prior slices made it difficultto detect pathological tissue or other irregularities in tissuestructure. It is difficult to compensate for the great amount of normaltissue included in a slice of 8 millimeters thickness when the data fromthe slice is analyzed. As a result, displayed representations of theslice, either visual or numerical, are not entirely satisfactory. Theaveraged normal tissue tends to obscure the presence of abnormalstructure.

The present device has considerable advantages over the prior art. Thenew device is continuously rotatable in one direction. Consequentlythere is no need to reposition or redjust cables for each rotation andthe examination proceeds considerably faster. In addition, the subjectis automatically repositioned with each complete rotation so anexamination sequence of several rotations and examination "slices" canbe conducted in sequence continuously. The x-ray source is pulsed manytimes as the x-ray source and detector are rotated around the subject.Each pulse constitutes an examination of the subject. One rotation ofthe device then provides a great quantity of data which can be analysedto provide a more accurate model of the examined subject. The x-rays areprojected in a thin beam, allowing greater discrimination betweenobserved structure, as described in the reference Neurological CATSystem by Artronix Incorporated, incorporated herein by reference.

Even with the thinner scan, the time of a complete examination sequencefor a subject such as a human head is considerably reduced. The time foreach scan is reduced due to the continuous mode of operation of thedevice and the pulsed series of examinations in each rotation. Thesubject is automatically indexed forward at the completion of eachrotation of the device and a new scan proceeds. As a result of theincreased speed per scan and the increased speed between scans in asequence, the total examination time for a subject is greatly reduced,with greater accuracy. As a result a tomograph machine can be used toexamine more subjects, such as human patients, per day with less cost incapital investment and labor per examination.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of the x-ray tomograph;

FIG. 2 is a longitudinal sectional view of the tomograph shown in FIG.1;

FIG. 3 is a cross sectional view of the tomograph taken along the planeof lines 3--3 in FIG. 2;

FIG. 4 is a cross sectional view of the tomograph taken along the planeof lines 4--4 in FIG. 2;

FIG. 5 is a partial cross sectional view of the tomograph taken alongthe plane of lines 5--5 in FIG. 4;

FIG. 6 is a partial cross sectional view of the energy discriminationblocks for the tomograph taken along the plane of lines 6--6 in FIG. 4;

FIG. 7 is a broken cross sectional view of the tomograph taken along theplane of lines 7--7 in FIG. 4 showing portions of the x-ray source, slitcollimator, head tank, scatter collimator, and detector;

FIG. 8 is a broken cross-sectional view of the scatter collimator takenalong the plane of lines 8--8 in FIG. 7;

FIG. 9 is a broken cross-sectional view of the detector taken along theplane of lines 9--9 in FIG. 7;

FIG. 10 is a broken sectional view of the scatter collimator anddetector;

FIG. 11 is a front plan view of a reference plate used in the detector;

FIG. 12 is a front plan view of a collector plate composite used in thedetector;

FIG. 13 is a cross-sectional view taken along the plane of lines 13--13in FIG. 12;

FIG. 14 is a cross-sectional view of the tomograph taken along the planeof lines 14--14 in FIG. 2;

FIG. 15 is a block diagram of the x-ray and scanner unit with supportingequipment;

FIG. 16 is a circuit diagram of the detector switching and amplifiercircuit;

FIG. 17 is a cross-sectional view of the power connection tank takenalong the plane of lines 17--17 in FIG. 2;

FIG. 18 is a longitudinal sectional view of the power connection tanktaken along the plane of lines 18--18 in FIG. 17; and

FIG. 19 is a circuit diagram of the x-ray pulsing and switching circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring in more detail to the drawings in FIG. 1, the complete axialx-ray tomograph 100 is shown. The tomograph 100 has a basic framestructure 102 which supports the tomograph. The tomograph 100 has amovable couch 104 supported in the frame 102. The couch 104 is connectedwith a shield 106 and a head tank 108 which receives the head of asubject to be x-rayed. Shield 106 overlaps a rotatable support 110 andthe head tank 108 projects through the center of the rotatable support110, as shown. Rotatable support 110 is mounted on ball bearings 112 ina mounting plate 114 attached to frame 102.

Couch 104 is mounted in frame 102 and driven by lead screw 116, shown inFIGS. 2 and 3. Lead screw 116 is driven by servo motor 118 which, inresponse to a signal, can reciprocally position the couch 104 alongsliding supports 119. Couch 104, shield 106 and head tank 108reciprocate relative to rotating support 110 with head tank 108positioned within opening 120 in the rotating support 110. Rotatingsupport 110 rotates concentrically about the horizontal axis of headtank 108 as is described herein.

Near an edge of rotating support 110 and along a diameter of support 110is mounted an x-ray tube 122, as shown, for rotation with rotatingsupport 110. Near the opposite edge of rotating support 110 andsubstantially bisected by the diameter on which x-ray tube 122 ismounted, is an x-ray detector 124, as shown. The rotating support 110 inthe area between the x-ray tube 122 and detector 124 is relieved, asshown in FIG. 4, to receive head tank 108. Surrounding head tank 108 isan acrylic field 126. Slit collimator 128 is spaced above acrylic field126 at a location between acrylic field 126 and the x-ray tube 122.Positioned between acrylic field 126 and detector 124 is scattercollimator 130.

Acrylic field 126 has two energy discrimination blocks 132 at the outeredges of field 126, as shown. Energy discrimination blocks 132 allowdetector 124 to measure the energy of the x-ray beam. The detector cellslying along the path of the beam passing through blocks 132 measure theamount of attenuation of the beam by blocks 132 and allow compensationfor the attenuation characteristics of the x-rays as a function of theenergy of the x-rays. Detector 124 may also contain suitable fieldstrength detectors, not shown.

On the side of rotatable support 110 opposite couch 104 and fixedcentrally to the rotatable support 110 and concentric about head tank108 is drive boss 134. Drive boss 134 is driven through belt 136 by thecombination of motor 138, transmission 140 and sheave 142. Drive boss134 connects to power housing 144 by bearings 146 and rotatable seal 148so that the boss 134 can rotate while power housing 144 remainsstationary. Power housing 144 contains rotating slip ring electric powerconnectors 150, 152 and 154 which rotate with rotating boss 134 androtatable support 110.

Slip rings 150, 152 and 154 are of non-conducting material, e.g., anacrylic or other suitable plastic, and have conducting metal rings 156on their periphery. Power for x-ray tube 122 is supplied throughspring-biased contacts 158 and cables 160, 162 which pass through thewall 163 of power housing 144. Metallic conductors 164, 166 and 168conduct the power through seals 170, 172 and the back wall 174 ofrotating boss 134 and then via cables 176, 178 to x-ray tube 122.

Additional conductors 180 pass centrally through power housing 144through a sealed low power slip ring connection 182, as shown.Conductors 180 supply connections to the control mechanism, such as adigital computer, e.g., an Artronix, PC12 (TM) which controls theoperation and functions of the tomograph. Conductors 180 supply thepower connections to the detector 124 and for certain auxiliaryfunctions of the tomograph, if desired, as will be appreciated by thoseskilled in the art. Power housing 144 is filled with transformer oil,not shown, to prevent arcing of the high voltage power supply to thex-ray tube, and has a sight glass 184 for the transformer oil which alsoserves as an expansion chamber.

The structure of x-ray source 122, slit collimator 128, head tank 108,scatter collimator 130 and detector 124, are shown in more detail inFIGS. 7-12. X-ray tube 122 is of conventional design, but is equippedwith a lead disc primary collimator having a slit in the form of a leaddisc having a rectangular slot window 186 through which the emittedx-rays can pass in roughly a fan-shaped planar beam. The beam passesthrough primary shield 187 into the adjoining rectangular slitcollimator box 128, which is lined with lead 188, and which has a slit190 which again collimates the x-ray beam into a thin planar fan-shapedbeam. The beam passes through the adjacent acrylic field 126 mounted onrotatable support 110 and through acrylic head tank 108 receivedtherein. Head tank 108 is filled with water in use. The water isretained in head tank 108 by a latex rubber cap 192 which fits closelyaround the head of the subject.

From the object the x-ray beam passes through the bottom of head tank108 and through slit 194 into scatter collimator 130, mounted onrotatable support 110, as shown. Scatter collimator 130 has a leadlining 196 and scatter collimator plates 198 which lie radially alongthe x-ray beam. The source of the x-rays in x-ray tube 122 is the centerof curvature for the radii on which plates 198 lie. As shown, thescatter collimator 130 contains one hundred thirty-two collimator plates198. The angle between adjacent collimator plates 198 is 0.342°. Scattercollimator 130 absorbs scattered radiation passing through the subjectand allows only radiation which has been collimated to lie on radiiextending from the source of the radiation in x-ray tube 122 and in theplane of thin slot 200 at the base of scatter collimator 130 to enterslot 202 of detector 124. Detector 124 is mounted on rotatable support110 adjacent to scatter collimator 130, as shown. The collimatedradiation then passes through window 204 in detector 124 where it isabsorbed by xenon or other conventional ionizable gases, and is detectedon plates 210 of detector 124 as charged ions.

The structure of the tomograph, apart from the pulsing means claimedherein, is not claimed in the instant application but is describedherein for purposes of illustration of the environment in which thestructure disclosed and claimed may be used. The various structure ofthe tomograph is disclosed and claimed in the co-pending applications ofGregory A. Davis, Kenneth E. Krippner, Jan A. Roestel, Gottfreid Vonk,and Albert R. Zacher, Jr., entitled AXIAL TOMOGRAPHIC APPARATUS filedNov. 28, 1975, co-pending herewith Ser. No. 636,102; Albert R. Zacher,Jr., entitled AXIAL TOMOGRAPHIC APPARATUS AND DETECTOR, filed Nov. 28,1975, co-pending herewith Ser. No. 636,104; and Albert R. Zacher, Jr.and Kenneth E. Krippner, entitled X-RAY DETECTOR, filed Nov. 28, 1975,and co-pending herewith Ser No. 636,101.

The detector 124, as is scatter collimator 130, is roughly a truncatedfan-shaped box. Detector 124, as shown, also contains one hundredthirty-two plates. The plates intersect the plane of the x-ray beam andlie along radii aimed at the point source of the x-ray beam as shown.Detector 124 contains an atmosphere of xenon, not shown, at tenatmospheres absolute pressure. Due to the mechanical requirements ofconnecting the detector and the presence of other detector components,normally only one hundred twenty-eight measurements are taken by thecells defined by the plates 206, 208, 210.

The plates in detector 124 are an alternating series of plates 206 and208. Plates 206 are reference plates and are maintained at a potentialof minus 3 kilovolts relative to ground potential. Plates 208 also arereference plates and are maintained at ground potential. As shown inFIG. 12, Plates 208 have adhered thereon a thin polyethyleneterephthalate (Mylar TM) plate 210 on which there is a thin coating ofvaporized aluminum. However, any thin conductor, such as copper foil orits equivalent on an insulating substrate would be satisfactory. Plates210 are connected, through a series of sequentially firing electronicswitches 212, to an integrating amplifier circuit 214. Amplifier circuit214 is connected via wiring hardware, not shown, to the controllingswitching station, but preferably to an Artronix PC-12(TM) digitalcomputer, or other conventional computer. FIG. 16 shows the amplifiercircuit for the signals received from plates 210.

As shown in phantom in FIG. 4, the detector plates 206 and 208 lie alongradii 226 which have the point source of the x-radiation as their commoncenter of curvature. The scale of the plate placement is enlarged inFIG. 4 for greater clarity. As is also shown in FIG. 4, the plates arelocated in the detector so that a radius 228 which passes through theaxis of rotation of rotatable support 110, and thus through the centerof the object examined in head tank 108, does not lie along any ofplates 206 and 208.

The detector 124 and plates 206, 208 are located so that the radius 228,generated at the point source of the x-radiation in x-ray tube 122 andpassing through the axis of rotation of rotatable support 110 andthrough the center of the object in tank 108, passes between plates 206and 208 at a location which does not bisect the distance between plates206 and 208. The preferred location for radius 228 is one-fourth thedistance between plates 206 and 208, as shown.

By spacing the plates so that radius 228 passes one-fourth the distancebetween plates 206 and 208 greater spatial resolution is obtained incollecting the data in a single scan. The detector spaces betweenadjacent plates scan beams which intercept adjacent toroidal areas ofthe object and which progressively overlap from edge to center of theobject. By spacing the detector plates as described, the area of theobject scanned by any one detector space, also is overlapped during asingle rotation. In effect, a single scan of 360° rotation will takeoverlapping observations of the observed slice during one rotationgiving resolution equivalent to a scan of 180° rotation with detectorsspaced at intervals one-half as great. The finer resolution of theobservations increases the accuracy of subsequent analysis of the data.If the detector plates are placed symetrically about the radius whichpasses through the center of the object, one-half of the detector isredundant over a 360° scan. There is duplication of data, rather thanoverlap, and resolution is only one-half as good.

FIG. 15 shows the general layout of the tomographic scanner and itsauxiliary equipment such as the controlling digital computer, forexample, an Artronix PC-12(TM), or other conventional digital computer,suitably programmed.

As shown in FIG. 2, additional equipment included in the tomograph are awater pump 216 connected to a reservoir 218 via tubing 220 and also tohead tank 108 via tubes 222 and 224.

The x-ray generator 230, as shown in FIGS. 15 and 19, controls thepulsing of x-ray tube 122. X-ray generator 230 has a transformer 232 anda direct current biasing circuit 234 to bias the core 235 of transformer232 to prevent magnetic saturation of the transformer core 235.

As shown schematically in FIG. 19, an auto transformer 236 receivessixty cycle line voltage to compensate for variations in the linevoltage. When mechanical switch 238 is closed (when the machine 100 isactivated), biasing circuit 234 applies a substantially level directcurrent across the transformer primary windings 239, as shown by arrow240. The filter capacitor 242, surge limiting resistor 244, and currentlimiting resistor 248 are preferably chosen to provide a current equalto the transformer magnetization current and of opposite sign to thecurrent of the first half cycle. On initiation of operation of themachine 100, rotation of rotatable support 110 is started and after thebiasing circuit has been connected across the primary windings 239 ofthe high voltage transformer 232, the control mechanism (preferably theArtronix PC-12 TM computer) signals electronic switch 250 to fire thex-ray tube 122. The 60 cycle line voltage is conducted to the primarywindings 239 of the transformer 232 initially in the direction oppositeto biasing current provided by biasing circuit 234, as shown by arrow249. The electronic switch 250 closes for one complete cycle, allowingone cycle of voltage across primary windings 239 of high voltagetransformer 232, which is transformed into a one cycle high voltageoutput in the secondary windings 252 of the transformer and is conductedto x-ray tube 122 to provide a short pulse of x-rays.

The biasing circuit 234 assists in reducing the total current across theelectronic switch 250 so that the switch will disconnect the primarywindings 239 of transformer 232 from the line voltage. The switch 250then waits until a subsequent signal is sent by the controllingmechanism, the computer, that is, for one complete cycle.

The use of biasing circuit 234 is also valuable, since if high voltagetransformer 232 is cycled repeatedly without the bias, the core 235 willbecome magnetically saturated. If the core 235 becomes saturated, itwill not pass any additional voltage into the secondary windings 252 ofthe transformer 232 and the transformer 232 would fail to pulse thex-ray tube 122. The effective resistance across the primary windings 239would become very small and the current across the primary windings 239would then become dangerously high. Biasing circuit 234 prevents thesaturation of the core 235 of the transformer 232 by creating an initialmagnetization of opposition polarity and by allowing sufficient timebetween cycles for the magnetization of core 235 to equilibrate.

Other means for pulsing x-ray tube 122 may be used if desired, providedthat a suitable rapid pulsing and scanning is obtained.

OPERATION OF THE DEVICE

It will be appreciated that various operations of the x-ray tomographcould be controlled by an operator who initiates each function by aseries of conventional electrical switches connected to motor drives andservo motors, or by relay switches on the rotating support 110 and otherportions of the machinery, which on completion of a rotation energizevarious servo circuits. However, it is preferred that the entirefunction of the machine, once started, be controlled by a suitablecomputer program through a conventionally programmable digital computer,such as the Artronix PC-12(TM) or other conventional computer. Asuitable program is currently available and is used for this purpose byArtronix, Incorporated of St. Louis, Missouri, but one skilled in theart would easily devise a suitable program from the followingdescription of the machine functions, for example, by using FORTRANprogramming techniques such as are taught in Introduction to FORTRAN, byDaniel McCracken (McGraw-Hill).

To examine a patient, using the apparatus disclosed and claimed herein,the patient is positioned in a prone position as shown in phantom inFIG. 3, with the patient's head firmly fixed inside latex cap 192. Latexcap 192 expands, when pump 216 pumps water out of head tank 108 intoreservoir 218, thus creating a partial vacuum in head tank 108, makinginsertion of the patient's head a simple matter. When the head isproperly positioned inside latex cap 192, water is pumped from reservoir218 back into head tank 108 collapsing latex cap 192 closely around thesubject's head.

When the subject is suitably positioned in the device, the motor 138 isenergized starting rotation of rotating support 110 and high voltagepower is supplied to the x-ray tube 122. The x-ray tube is pulsed forvery short durations 256 times in the course of one complete 360°rotation. Normally one rotation will be completed in nine seconds orless. At the end of one complete rotation, the servo motor 118 isenergized indexing the patient forward, that is in a direction to theright as shown in FIG. 2. The patient is moved a distance ofapproximately 3 millimeters, the approximate width of the x-ray beam,and a new scan is started. Depending on the power used in the x-ray beamand the time between scans required to cool the x-ray equipment, as manyas three or more discrete full power examinations of 360° each can becompleted within one minute. This is much faster than conventionalequipment which may require about 5 minutes to complete two scans. Atypical examination of a patient can require less than 5 minutes.

Since the detector takes one hundred twenty-eight measurements per pulseand the x-ray source is pulsed from 256 separate positions in one 360°rotation of the tomograph, there are 32,768 total measurements taken ineach 360° rotation of the device. The value measured is the attenuationof the x-ray beam which passes into the detector. These attenuationmeasurements are subjected to non-cartesian (polar) analysis by means ofthe digital computer in a 256 × 256 matrix, so that each word of datarepresents the absorbtion value for a volume of analyzed tissue that is1 millimeter by 1 millimeter by 3 millimeters (the thickness of thex-ray beam). While it would be possible to use the successiveapproximation technique disclosed by U.S. Pat. No. 3,778,614, thespecification of which is included herein by reference, it is preferredthat an algebraic transformation be used which takes advantage of thenon-cartesian geometry of this measurement system. A suitabletransformation is that of A. M. Cormack, "Representation of a Functionby its Line Integrals, With Some Radiological Applications II,"published in the Journal of Applied Physics, v. 35, No. 10, October,1964.

The attenuation of the x-ray beam is preferably measured on an arbitraryscale in which the attenuation of water is zero; the attenuation of airis -500 and the attenuation of dense bone is approximately +500. Theattenuation of the acrylic head tank 108 and field 126 are alsoessentially zero, as is the value for normal soft tissue. This scaleimproves the precision of the computation in analyzing the dataobtained, since a simple null comparison can be made and any deviationfrom the null analyzed.

The strength of the attenuated radiation is measured by detector 124.After the radiation from x-ray tube 122 is collimated into a thinfan-shaped beam by slot 186 and slit collimator 128 it passes throughthe subject and through acrylic head tank 108, through the surroundingwater, not shown, in head tank 108, and through field 126. The beam iscollimated in collimator 130 to eliminate the scattered radiation andpasses into the detector 124. There the x-rays ionize the xenon gas, notshown, present in the detector 124. The strength of the charge of theionized gas is proportional to the strength of radiation passing intothe detector 124. The ionized xenon travels to plates 210 where thecharge is read through switches 212 and integrator-amplifier circuit 214and digitized by an analog to digital converter, not shown. The digitalvalue is stored in a digital computer or other device. The switches 212are set to fire sequentially so that only one amplifier circuit 214 isneeded to transmit the data into computer storage.

Energy discrimination blocks 132 enable the dectector 124 to measure theenergy of the x-ray beam so that the attenuation characteristics of themater x-ray beam can be calculated and small variations and artifactscan be eliminated. The energy attenuation function of x-rays is wellknown to those skilled in the art and the computer program having theattenuation characteristics built in could be easily produced by thoseskilled in the art. Such a program is currently produced by Artronics,Inc.

The data obtained is analyzed mathematically by operation of thecomputer and can be printed out in numerical form and stored byconventional means, either tape or disc, for on-line printers andsimilar equipment. Additional peripheral or storage equipment, i.e.,video, can be provided, as is well known to those skilled in the art.The data can be collected by full power scanning of individual slicesand displayed per individual slice or can be collected on low powerscanning and a plurality of slices averaged and analyzed to produce onedisplay, as required by the particular radiological examination beingconducted.

It will be appreciated by those skilled in the art that many othervariations and modifications may be made of the apparatus and methodsdisclosed herein without departing from the spirit of the invention. Itis expected that the invention is not to be limited by the disclosureincorporated herein for purposes of illustration, but is limited only bythe scope of the appended claims.

I claim:
 1. An apparatus for examining an object by radiation having asource of radiation and means to generate radiation from that source,the means to generate radiation including a high voltage transformer tosupply a pulsed, high-voltage output to the radiation source to rapidlypulse the radiation generated from the source of radiation, the highvoltage transformer having means to pulse a low voltage input to thehigh voltage transformer for one cycle of the input voltage and havingmeans to apply a low direct current bias to the high voltagetransformer.
 2. The device of claim 1 wherein the device has means tocontinuously apply the low direct current bias to the transformer andmeans to shut off the low voltage input to the transformer for a cyclewhile the bias is applied.
 3. The device of claim 2 wherein the means toapply the low direct current bias applies a bias of opposite sign to theinitial sign of the current produced by the pulsed voltage.
 4. Thedevice of claim 1 wherein the device has means to rotate the source ofradiation about the object to be examined.
 5. The apparatus of claim 1wherein the means to pulse the source of radiation has means for rapidand continuous sequential pulsing.
 6. The device of claim 1 wherein themeans to pulse the source of radiation has a switch means to pulse analternating low voltage input to the primary windings of the highvoltage transformer.
 7. In an apparatus for examining an object byradiation having a source of radiation and wherein the source ofradiation is rotated about the object to be examined, the improvementcomprising means for rapidly pulsing the source of radiation, the meanshaving a high voltage transformer with means to supply a high voltageoutput to the source of radiation, the pulsing means having a switchmeans to pulse an alternating low voltage input to the primary windingsof the high voltage transformer for one complete cycle of input voltage,and means to apply a direct current bias to the transformer primarywindings of opposite sign to the initial sign of current produced by thepulsed voltage, to prevent saturation of the core of the high voltagetransformer during pulsing, the direct current bias inducing an initialmagnetization of the transformer core of opposite polarity to that whichis induced by the initial phase of the pulsed voltage, wherebysaturation of the core of the high voltage transformer is preventedduring pulsing and rapid and continuous sequential pulsing of theradiation source is possible.